The invention relates to a hydraulic brake system as defined hereinafter.
Hydraulic brake systems, as they are used in motor vehicles, have a master brake cylinder that is mechanically actuated by a brake pedal, causing the buildup of a corresponding brake pressure in the brake lines leading to the wheel brake cylinders. To prevent locking of the wheels, or skidding, so-called anti-lock or anti-skid brake systems (ABS) are known, in which the brake pressure is reduced as soon as skidding is detected or is at least imminent. To this end, a hydraulic cylinder disposed between the master brake cylinder and the wheel brake cylinder can be provided, the piston of which, by mechanical actuation, effects a lowering or modulation of brake pressure. The additional hydraulic cylinder may also be disposed such that an increasing of brake pressure is attained when it is mechanically actuated. In that case, the hydraulic cylinder functions as a brake booster. A buildup of brake pressure at the driven wheels that is independent of the driver offers the opportunity for anti-slip regulation (ASR).
U.S. Pat. No. 4,083,609 discloses a hydraulic cylinder the piston of which is displaceable via its piston rod by a positioning mechanism embodied as an electromagnet. However, the use of an electromagnet as the positioning mechanism has the disadvantage that the system is relatively sluggish and is unsuited to relatively large piston deflections and cannot bring to bear the great forces appropriate for the pressures involved.